The reduction reaction of nitrate by metallic iron under aerobic conditions was investigated in the pH range of 2.0–8.5. The concentration of nitrate, nitrite, ammonia/ammonium and total nitrogen in the reduction solution were monitored. The reduction products of nitrate were ammonia/ammonium remaining in the reduction solution and gaseous nitrogen emitted from the reduction solution. Nitrate predominantly reduced to gaseous nitrogen at the beginning of the reduction reaction, and ammonia/ammonium at the end of the reduction reaction. pH value was an important parameter affecting the reduction of nitrate by metallic iron. At a low pH values, ammonium was the main reduction product, while gaseous nitrogen was the dominant reduction product at high pH values. In the reduction products, gaseous nitrogen was 40.1, 56.0, 68.0, 77.8 and 81.0% after 150-min reduction at pH=2.0, 3.5, 5.0, 7.0 and 8.5, respectively.